Clamping retainer for chemical mechanical polishing

ABSTRACT

A carrier head for chemical mechanical polishing includes a housing, a substrate mounting surface, and a retaining ring assembly. The retaining ring assembly includes an inner ring surrounding the substrate mounting surface and having an inner surface to retain the substrate below the substrate mounting surface, a first actuator to adjust a vertical load on the inner ring, an outer ring surrounding the inner ring, and a second actuator positioned between the inner ring and the outer ring. The inner ring has a plurality of slots that are formed in a lower surface and that extend from the inner surface to an outer surface of the inner ring to divide the inner ring into a plurality of arcuate segments suspended from an upper portion. The second actuator applies a radially inward pressure such that the plurality of arcuate segments flex inwardly relative to the upper portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Application No.63/346,802, filed on May 27, 2022, the contents of which are herebyincorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a retainer for use in chemicalmechanical polishing of substrates and a method of operating such aretainer.

BACKGROUND

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. One fabrication step involves depositing afiller layer over a non-planar surface and planarizing the filler layer.For certain applications, the filler layer is planarized until the topsurface of a patterned layer is exposed. A conductive filler layer, forexample, can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. After planarization, theportions of the conductive layer remaining between the raised pattern ofthe insulative layer form vias, plugs, and lines that provide conductivepaths between thin film circuits on the substrate. For otherapplications, such as oxide polishing, the filler layer is planarizeduntil a predetermined thickness is left over the non planar surface. Inaddition, planarization of the substrate surface is usually required forphotolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is typically placed against a rotating polishing pad.The carrier head provides a controllable load on the substrate to pushit against the polishing pad. An abrasive polishing slurry is typicallysupplied to the surface of the polishing pad.

The carrier head provides a controllable load on the substrate to pushit against the polishing pad. A retaining ring is used to hold thesubstrate in place below the carrier head during polishing. Some carrierheads apply pressure to urge the retaining ring into contact with thepolishing surface.

SUMMARY

In one aspect, a carrier head for chemical mechanical polishing includesa housing, a substrate mounting surface, and a retaining ring assembly.The retaining ring assembly includes an inner ring surrounding thesubstrate mounting surface and having an inner surface to retain thesubstrate below the substrate mounting surface, an outer ringsurrounding the inner ring, and an actuator positioned between the innerring and the outer ring. The inner ring has a lower surface and aplurality of slots that are formed in the lower surface and that extendfrom the inner surface to an outer surface of the inner ring to dividethe inner ring into a plurality of arcuate segments suspended from anupper portion. The actuator applies a radially inward pressure such thatthe plurality of arcuate segments flex inwardly relative to the upperportion.

In another aspect, a method of polishing includes bringing a substrateinto contact with a polishing surface, generating relative motionbetween the substrate and the polishing surface, and applying a radiallyinward pressure to the substrate by pressing inwardly on an inner ringthat has a plurality of slots that divide the inner ring into aplurality of arcuate segments suspended from an upper portion.

In another aspect, a method of polishing includes bringing a substrateinto contact with a polishing pad and generating relative motion betweenthe substrate and the polishing pad, retaining the substrate on thepolishing pad with a retainer, and during polishing of the substratealternating between reducing a diameter of an inner surface of theretainer to clamp the substrate and increasing the diameter of the innersurface of the retainer to release the substrate from clamping whilecontinuing to retain the substrate.

In another aspect, a polishing system includes a support to hold apolishing pad, a carrier head to hold the substrate against thepolishing pad, and a controller. The carrier head includes a firstchamber to apply a first downward pressure to a center portion of thesubstrate held by carrier head, a second chamber to apply a seconddownward pressure to an outer portion of the substrate surrounding thecentral portion, and an inner surface to engage an edge of thesubstrate. The inner surface has an adjustable diameter. The controlleris configured to, in response to identifying a polishing non-uniformity,decrease the diameter of the inner surface of the retainer and selectwhether the first pressure is greater or lower than the second pressureso as to reduce the polishing non-uniformity.

In another aspect, a polishing system includes a support to hold apolishing pad, a carrier head to hold the substrate against thepolishing pad, and a controller. The carrier head includes a firstchamber to apply a first downward pressure to a center portion of thesubstrate held by carrier head, a second chamber to apply a seconddownward pressure to an outer portion of the substrate surrounding thecentral portion, and an inner surface to engage an edge of thesubstrate. The inner surface has an adjustable diameter. The controlleris configured to, in response to identifying a polishing non-uniformitydecrease the diameter of the inner surface of the retainer sufficientlythat the substrate bows, determine whether the substrate should bowinwardly or outwardly from the carrier head to reduce the polishingnon-uniformity, and select whether the first pressure is greater orlower than the second pressure such that the substrate bows in thedetermined direction.

Implementations may optionally include, but are not limited to, one ormore of the following advantages. Distribution of force during polishingbetween the substrate and the retaining ring can be modified so thatforce is redistributed along the edge of the substrate. This distributedcontact force can reduce local wafer deformations and can improve theoperator's ability to control substrate edge removal profile. Polishingnon-uniformity, e.g., caused by a polishing head profile issue at asubstrate edge, can be reduced. The retaining ring can be operated athigher clamping force in concert with the pressure from the membrane ofthe carrier head to change the shape of the substrate, which can modifypolishing rates across the substrate.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other aspects,features, and advantages will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a carrier head in a chemicalmechanical polishing system.

FIG. 2A shows a cross-sectional side view of the retainer assembly ofFIG. 1 .

FIG. 2B shows a bottom view of the retainer assembly of FIG. 2A.

FIG. 2C shows a cross-sectional side view of the retainer assembly ofFIG. 2A in an actuated configuration.

FIG. 3 shows a cross-sectional side view of another implementation of aretainer assembly.

FIG. 4 shows a cross-sectional side view of yet another implementationof a retainer assembly.

FIG. 5 shows a cross-sectional side view of still another implementationof a retainer assembly.

FIGS. 6A and 6B are schematic illustrations showing pressures applied tobow a substrate into a convex or concave configuration, respectively.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Some carrier heads include a retaining ring with a circular innersurface to retain the substrate. Typically the inner diameter of theretaining ring is slightly larger than the diameter of the substrate,e.g., by 1-3 mm. In this configuration the substrate can rotate relativeto the carrier head and retaining ring; this relative movement is termed“precession.”

Precession can be useful for reducing asymmetric non-uniformities.

During polishing frictional forces can drive the substrate edge againstthe inner surface of the retaining ring. A potential problem with acircular inner surface is that the force from the substrate can beconcentrated at a single point of contact between the substrate and theretaining ring, which can lead to scratching or other damage to theinner surface, or to unintended warping of the substrate near the pointof contact, which can induce polishing non-uniformities.

A retaining ring with a flexible inner surface or with an adjustableinner diameter has been proposed. Hypothetically such a configurationwould permit the retaining ring diameter to be reduced so that thesubstrate contacts along an extended region rather than at a singlepoint. However, such a configuration has apparently not beencommercialized. Thus, there remains room for improvement on design of aretaining ring having an adjustable inner diameter.

FIG. 1 illustrates an example of a polishing station of a chemicalmechanical polishing system 20. The polishing system 20 includes arotatable disk-shaped platen 24 on which a polishing pad 30 is situated.The platen 24 is operable to rotate about an axis 25. For example, amotor 26 can turn a drive shaft 28 to rotate the platen 24. Thepolishing pad can be a two-layer polishing pad with an outer polishinglayer 32 and a softer backing layer 34.

The polishing system 20 can include a supply port or a combinedsupply-rinse arm 36 to dispense a polishing liquid 38, such as anabrasive slurry, onto the polishing pad 30. The polishing system 20 caninclude a pad conditioner apparatus 40 with a conditioning disk 42 tomaintain the surface roughness of the polishing pad 30. The conditioningdisk 42 can be positioned at the end of an arm 44 that can swing so asto sweep the disk 42 radially across the polishing pad 30.

A carrier head 70 is operable to hold a substrate 10 against thepolishing pad 30. The carrier head 70 is suspended from a supportstructure 50, e.g., a carousel or a track, and is connected by a driveshaft 54 to a carrier head rotation motor 56 so that the carrier headcan rotate about an axis 58. Optionally, the carrier head 70 canoscillate laterally, e.g., on sliders on the carousel, by movement alongthe track, or by rotational oscillation of the carousel itself.

The carrier head 70 includes a housing 72, a substrate backing assembly74 which includes a base 76 and a flexible membrane 78 that defines aplurality of pressurizable chambers 80, a gimbal mechanism 82 (which maybe considered part of the assembly 74), a loading chamber 84, aretaining ring assembly 100, and an actuator 122.

The housing 72 can generally be circular in shape and can be connectedto the drive shaft 54 to rotate therewith during polishing. There may bepassages (not illustrated) extending through the housing 72 forpneumatic control of the carrier head 70. The substrate backing assembly74 is a vertically movable assembly located beneath the housing 72. Thegimbal mechanism 82 permits the base 76 to gimbal relative to thehousing 72 while preventing lateral motion of the base 76 relative tothe housing 72. The loading chamber 84 is located between the housing 72and the base 76 to apply a load, i.e., a downward pressure or weight, tothe base 76 and thus to the substrate backing assembly. The verticalposition of the substrate backing assembly 74 relative to a polishingpad is also controlled by the loading chamber 84. The lower surface ofthe flexible membrane 78 provides a mounting surface for a substrate 10.

In some implementation, the substrate backing assembly 74 is not aseparate component that is movable relative to the housing 72. In thiscase, the chamber 84 and gimbal 82 are unnecessary.

Referring now to FIGS. 1 and 2A, the retaining ring assembly 100includes an inner ring 110, a first actuator 120 to adjust a verticalheight of or downward pressure on the inner ring 110, an outer ring 130,and a second actuator 140 between the inner ring 110 and the outer ring130 to adjust a radially inward directed pressure on the inner ring 110.A lower surface 112 of the inner ring 110 can contact the polishing pad30. Similarly, a lower surface 132 of the outer ring 130 can contact thepolishing pad 30.

The inner ring 110 is an annular body that is vertically movablerelative to the housing 72. The inner ring 110 has an inner surface thatis configured to circumferentially surround the edge of the substrate 10to retain the substrate 10 in the carrier head during polishing. Theinner surface of the inner ring 110 can be a vertical cylindricalsurface that extends from the lower surface 112 to the upper annularsurface.

An outer surface of the inner ring 110 can optionally include a lip 114at the lower surface 112 that projects outwardly from a cylindricalportion toward the outer ring 130. The lip 114 can abut the outer ring130 to restrain movement of the inner ring 110 without inducingsignificant torque out of the plane of the polishing surface. In someimplementations, the inner ring includes a lower portion 110 b formed ofa wearable material, e.g., a plastic, and an upper portion 110 a formedof a more rigid material, e.g., a metal.

Referring to FIG. 2B, the inner ring 110 includes multiple radial slots116 that extend from the bottom surface 112 upward past the actuator140. Thus the inner ring 110 is divided by into multiple arc segments118 a that extend downwardly from a circular upper portion 118 b. Asdiscussed below, these arc segments 118 a are independently flexiblerelative to the connecting circular upper ring 118 b. Each arc segment118 a can extend through an arc (relative to the center of the innerring 110) of 5-20°. The slots 116 can be uniformly spaced at equalangular intervals around the inner ring 110.

Returning to FIG. 2A, the first actuator 120 can be a pressurizablechamber. For example, an annular membrane 122 can have an outer lipclamped to the housing 72 and an inner lip clamped to a top surface ofthe inner ring 110 to form a chamber 126. Alternatively, the firstactuator can be provided by an inflatable bladder, or by a linear motoror piezoelectric actuator.

The outer ring 130 is vertically fixed relative to the housing 72, andis an annular body that provides positioning or referencing of thecarrier head 70 to the surface of the polishing pad 30. In addition, theouter ring 130 provides lateral referencing of the inner ring 110against the polishing pad 30. The outer ring 130 circumferentiallysurrounds, e.g., is coaxial with, the inner ring 110.

The outer ring 130 has an outer surface, which can be a verticallycylindrical surface. The vertical cylindrical outer surface can extendupwardly from an outer edge of the lower surface 132. The outer ring 130also has an inner surface that is separated by a gap 134 from the outersurface of the inner ring 110. In some implementations, the outer ringincludes a lower portion 130 b formed of a wearable material, e.g., aplastic, and an upper portion 130 a formed of a more rigid material,e.g., a metal. In some implementations, the entirety of the inner ring110 is formed of a material that is more flexible than the upper portion130 a of the outer ring. For example, the inner ring 110 can be formedof the same material as the lower portion 130 b of the outer ring 130.

The outer ring 130 can be secured to the housing 72, for example, by anadhesive, a fastener, or by interlocking parts. For example, an uppersurface 136 of the outer ring 130 can include cylindrical recesses orholes with screw sheaths (not shown) to receive fasteners, such asbolts, screws, or other hardware. For example, a fastener, such as ascrew or bolt, can extend through the housing 72 to secure the outerring 130 of the retaining ring assembly 100 to the housing 72.

The inner ring 110 can be relatively narrow as compared to the outerring 130. For example, the inner ring 110 can have a width W of 1-10 mm,e.g., 1-3 mm, e.g., 2 mm. The width W can be measured at the narrowsection of the inner ring 1110, e.g., above the lip 114.

The second actuator 140 is positioned between the inner surface of theouter ring 130 and the outer surface of the inner ring 110. The secondactuator 140 can be an inflatable annular bladder; pressurization of thebladder inflates the bladder and exerts a radially inwardly directedpressure on the arc segments 118 a of the inner ring 110. Alternatively,the second actuator can be provided by an inflatable bladder, or by alinear motor or piezoelectric actuator. In any event, as shown in FIG.2C, actuation by the second actuator 140 causes the arc segments 118 ato flex inwardly relative to the upper ring portion 118 b. As a result,the effective diameter of the inner surface of the inner ring 110 thatcontacts the substrate is reduced. Sufficient reduction of the diameter,e.g., by 1-3 mm, can cause the inner ring 118 to “clamp” the substrate,i.e., establish contact along the entire circumference of the substrate.

For pneumatic control of the second actuator 140, a pneumatic controlline 92 can extend from the bladder to a controllable pressure source94. The control line 92 can be provided by a combination of passagesthrough solid parts, piping, tubing, etc. The control line 92 can extendthrough the housing 72, and the drive shaft 54, and be connected to thepressure source 92 by a rotary coupling.

FIG. 2A illustrates that the second actuator 140 fits at least partiallyinto a recess 138 in the inner surface of the outer ring 130. However,there could be a recess in the outer surface of the inner ring, or bothrings could have aligned recesses, to accommodate the second actuator140.

The arc segments 118 a of the inner ring 110 can flex so the portionadjacent the lower surface 112 is horizontally movable relative to theouter section 144 when acted upon by the second actuator 140. Inparticular, when the second actuator 140 presses inwardly, the arcsegments 118 a flex inwardly, so the gaps between the arc segmentsnarrow and the effective diameter of the inner surface of the inner ring110 decreases.

Returning to FIG. 1 , inward pressure on the arc segments 118 a of theinner ring 110 by the second actuator 140 is controlled by a controller90. For example, the controller 90 can control the pressure applied bythe pressure source 94, e.g., by controlling a valve or the like. Thecontroller 90 can also control downward pressure on the inner ring 110by the first actuator 120, as well as other polishing system parameters,e.g., carrier head rotation rate, platen rotation rate, pressure ofchamber inside the carrier head, slurry dispensing rate, etc. Thecontroller 90 can be provided by a dedicated circuitry, a generalpurpose or programmable computer or application specific integratedcircuit that executes instructions stored in a non-transitory computerreadable medium.

FIG. 3 illustrates an implementation of the retaining ring assembly 100which is similar to that of FIG. 2A, but the inner ring 110 and outerring 130 are secured so as to be vertically fixed relative to eachother. The inner ring 110 and outer ring 130 together can be consideredto form a combined retainer 150. The combined retainer 150 can either bevertically movable relative to the housing 72 by the first actuator 120,e.g., as described above for the inner ring in FIG. 2A, or verticallyfixed to the housing 72, e.g., by adhesive, mechanical fasteners, etc.,as described above for the outer ring in FIG. 2A (in this case, there isno “first actuator”).

In some implementations, the inner ring 110 includes an outwardlyextending flange 152 that extends over the top surface of the outer ring130. The bottom of the flange 152 can be secured to the top surface ofthe outer ring 130, e.g., by adhesive, mechanical fasteners, etc.

In an unbiased state, the portions of the outer surface of the innerring 110 and the inner surface of the outer ring 130 located above thesecond actuator 140 are separated by a vertical cylindrical gap 134. Thegap 134 can be relatively narrow, e.g., 10-100 μm.

FIG. 4 illustrates an implementation of the retaining ring assembly 100which is similar to that of FIG. 3 , but instead of the inner ring 110having an outwardly extending flange, the outer ring 130 has an inwardlyextending flange 160 that extends over the top surface of the inner ring110. The bottom of the flange 160 can be secured to the top surface ofthe inner ring 110, e.g., by adhesive, mechanical fasteners, etc.

FIG. 5 illustrates an implementation of the retaining ring assembly 100which is similar to that of FIGS. 3-4 , but the second actuator 140 ispositioned to apply both an inward force on the outer surface of theinner ring 110, and a downward force on the upper surface of the lip114. This downward force will also tend to cause the arc segments 118 ato flex inwardly.

By reducing the effective diameter of the inner ring, and in particularby reducing the effective diameter until the substrate is clamped,lateral force of the substrate on the retainer is distributed across asignificant arc rather at a single point. This distributed contact forcebetween can reduce local wafer deformations, and can reduce thelikelihood of scratching and damage to the inner surface of theretainer. More generally, the inward pressure provides another “knob” toadjust the polishing profile, permitting greater flexibility and abilityto control the substrate edge removal profile.

A potential danger with clamping the substrate is that clamping canprevent the substrate from precessing relative to the carrier head.However, precession can reduce asymmetric (i.e., angularly varying)polishing non-uniformities. Thus, the controller 90 can be configured tooperate the second actuator so that the substrate is temporarilyreleased from clamping (but still retained) to allow precession and thenclamped again. In other words the controller can, while the substrate isbeing polished, cause the retaining ring assembly to alternate between afirst pressure at which the substrate is clamped and a second pressureat which the substrate is released and free to precess.

Referring to FIGS. 6A and 6B, the retaining ring assembly 100 can beoperated in conjunction with the pressurizable chambers 80 of thecarrier head 70 to deliberately change the shape of the substrate, e.g.,to cause the substrate to assume a convex (center bowing outward towardthe polishing pad) or a concave (center bowing inward away from thepolishing pad) configuration.

Referring to FIG. 6A, an inward clamping force (A) on the substrate edgewill tend to cause the substrate to bow. If the downward pressure (B)applied to the center of the substrate is greater than the downwardpressure (C) applied to the edges of the substrate (e.g., if thepressure in a center chamber is higher than a pressure in a surroundingouter chamber), then the substrate will tend to assume a convexconfiguration. This will cause the pressure of the polishing pad on thecenter of the substrate to increase, so the center polishing rate willincrease relative to edge polish rate.

In contrast, referring to FIG. 6B, if the downward pressure (B) appliedto the center of the substrate is less than the downward pressure (C)applied to the edges of the substrate (e.g., if the pressure in a centerchamber is lower than a pressure in a surrounding outer chamber), thenthe substrate will tend to assume a concave configuration. This willcause the pressure of the polishing pad on the center of the substrateto decrease, so the center polishing rate will decrease relative to edgepolish rate.

The controller 90 can be configured to cause the carrier head to applyappropriate pressures to the substrate so as to selectively cause thesubstrate to assume a concave or convex configuration. For example, ifthe controller 90 receives data from an in-situ monitoring system anddetects that the substrate edge is polishing faster than the substratecenter, the controller 90 can cause the downward pressure (B) applied tothe center of the substrate to be greater than the downward pressure (C)applied to the substrate edge so the center polishing rate will increaserelative to edge polish rate.

As used in the instant specification, the term substrate can include,for example, a product substrate (e.g., which includes multiple memoryor processor dies), a test substrate, a bare substrate, and a gatingsubstrate. The substrate can be at various stages of integrated circuitfabrication, e.g., the substrate can be a bare wafer, or it can includeone or more deposited and/or patterned layers. The term substrate caninclude circular disks and rectangular sheets.

The above described polishing system and methods can be applied in avariety of polishing systems. Either the polishing pad, or the carrierhead, or both can move to provide relative motion between the polishingsurface and the substrate. The polishing pad can be a circular (or someother shape) pad secured to the platen. The polishing layer can be astandard (for example, polyurethane with or without fillers) polishingmaterial, a soft material, or a fixed-abrasive material. Terms ofrelative positioning are used; it should be understood that thepolishing surface and substrate can be held in a vertical orientation orsome other orientation.

Particular embodiments of the invention have been described. Otherembodiments are within the scope of the following claims. For example,the actions recited in the claims can be performed in a different orderand still achieve desirable results.

What is claimed is:
 1. A carrier head for chemical mechanical polishing,comprising: a housing; a substrate mounting surface; and a retainingring assembly including an inner ring surrounding the substrate mountingsurface and having an inner surface to retain the substrate below thesubstrate mounting surface, the inner ring having a lower surface and aplurality of slots that are formed in the lower surface and that extendfrom the inner surface to an outer surface of the inner ring to dividethe inner ring into a plurality of arcuate segments suspended from anupper portion, an outer ring surrounding the inner ring, and a firstactuator positioned between the inner ring and the outer ring to apply aradially inward pressure such that the plurality of arcuate segmentsflex inwardly relative to the upper portion.
 2. The carrier head ofclaim 1, wherein the inner ring is independently vertically movablerelative to the outer ring.
 3. The carrier head of claim 2, wherein theouter ring is vertically fixed to the housing.
 4. The carrier head ofclaim 3, wherein the inner ring is suspended from the housing by asecond actuator.
 5. The carrier head of claim 1, wherein the inner ringis vertically fixed relative to the outer ring.
 6. The carrier head ofclaim 5, comprising a second actuator positioned between the housing andthe inner ring and the outer ring to adjust a vertical position of theinner ring and the outer ring.
 7. The carrier head of claim 6, whereinthe inner ring comprises a flange that extends between a top surface ofthe outer ring and the second actuator.
 8. The carrier head of claim 6,wherein the outer ring comprises a flange that extends between a topsurface of the inner ring and the second actuator.
 9. The carrier headof claim 1, wherein the first actuator comprises a bladder.
 10. Thecarrier head of claim 9, wherein an inner surface of the outer ring hasa first projection extending above the bladder.
 11. The carrier head ofclaim 10, wherein the inner surface of the outer ring has a secondprojection extending below the bladder.
 12. The carrier head of claim10, wherein an outer surface of the inner ring has a second projectionextending below the bladder.
 13. The carrier head of claim 9, whereinthe bladder is positioned at least partially in a recess in an outersurface of the inner ring.
 14. The carrier head of claim 1, wherein anentirety of the inner ring is a first plastic.
 15. The carrier head ofclaim 14, wherein the outer ring comprises an lower portion and a upperportion that is more rigid than the lower portion.
 16. The carrier headof claim 15, wherein the lower portion of the outer ring is the firstplastic.
 17. The carrier head of claim 1, wherein the slots are spacedat equal angular intervals around a center axis of the inner ring. 18.The carrier head of claim 1, wherein each arcuate segment extendsthrough an arc of 5-20° relative to the center of the inner ring.
 19. Amethod of polishing, comprising: bringing a substrate into contact witha polishing surface; generating relative motion between the substrateand the polishing surface; and applying a radially inward pressure tothe substrate by pressing inwardly on an inner ring that has a pluralityof slots that divide the inner ring into a plurality of arcuate segmentssuspended from an upper portion.
 20. The method of claim 19, comprisingmoving the inner ring independently vertically movable relative to anouter ring that surrounds the inner ring.